The invention pertains to electro-acoustical systems and more particularly to electro-acoustical systems for providing reverberant enhancement to rooms or spaces.
As is well-known in the acoustical art, a natural reverberant field, developing within a room, is characterized by diffusion, phase randomness and slight variations in amplitude at any point within the room during the decay of the reverberant field. The perception to a listener of such a naturally reverberant field is of being surrounded by an amorphous, directionless field of sound which decays in such a way as to include subtle sensations of animation and space.
A wide variety of methods are known for augmenting room acoustic reverberation by the use of electronic devices. Such electronic augmentation systems typically provide various types of delay lines, reverberation devices, and sound chambers. Multi-channel systems employing positive feedback for providing reverberation augmentation are also known.
However, with all prior known systems for electronically developing reverberation to enhance the reverberance of rooms, there have been shortcomings. For example, the perception of reverberance diffusion is often difficult to achieve with a necessarily limited number of loudspeakers, in comparison to the diffusion created by an extremely large number of virtual sources such as occurs in a natural reverberant field. The large number of virtual sources in a naturally occurring reverberant field is due to reflections from every direction.
Furthermore, in prior known electro-acoustical systems providing reverberation, the perceived quality of the reverberation is relatively unnatural. In such prior art systems, there is a perceived sense of insufficiency and a change of tonal quality as the natural field dissipates and the electronically augmented field continues. This sense of insufficiency is apparently caused by irregular frequency response, sensitivity to room modes, lack of enough perceived directionlessness, and lack of subtle animation effects that characterize the decay in the aforementioned natural reverberant fields.
In such known electro-acoustical systems, there has also been a tendency for the electronically enhanced reverberation to be overly sensitive to room modes, i.e., the reverberation tended to develop to an excessive degree in response to some tones sounded in a room and to develop to an insufficient degree in response to other tones. For example, this effect is particularly elicited by the pure tones of an instrument such as a pipe organ. As a direct result of such sensitivity, microphone placement can be extremely difficult and impractically critical.
Due partially to the aforementioned sensitivity to room modes and the attendant progressive disruption of the amplitude versus frequency response of the augmented reverberation into an array of steep hills and valleys, it has proven in the past to be difficult to develop both believable tonal quality and adequate level in electronically augmented reverberant fields.
The electronic tools of various electro-acoustical systems, particularly as applied to musical instruments, are well known. Reverberation devices, delay lines, and amplitude and phase modulators have all been applied in recording studio signal processing systems for making sound recordings. Such devices have also been applied in electronic organs and other electronic musical instruments. The hardware for such implementations is commonplace.
It has also been known in past electro-acoustical systems to provide phase modulation to direct positive acoustic feedback reverberation-extension units. For example, Guelke and Broadhurst describe, in "Reverberation Time Control by Direct Feedback", Acustica Vol. 24 (1971) at pages 33-41, phase modulation being applied to direct electro-acoustic feedback in order to increase the gain before feedback of a concert hall reverberation-extension system. In "Assisted Reverberation in an Outdoor Environment", Acustica Vol. 38 (1977) pages 335-337, Mr. Guelke describes a two-channel reverberation system at an outdoor concert location where phase modulation is applied. The stated goal of applying phase modulation in each of these papers was to increase level before the onset of instability. In the Guelke systems, phase modulation was applied in only one channel. Another known electro-acoustical system utilizing a reverberation chamber and a delay device for providing relative variation in magnitudes of the delayed energy increments in advance of the reverberation is described in U.S. Pat. No. 3,535,453, issued to Veneklasen (1970).
However, it has not heretofore been known to provide an electro-acoustical reverberance enhancement system which results in an increased sense of naturalness, directionlessness and animation through random phase variation between output and input. It has further not heretofore been known to provide such a system which may be adapted to function with a large number of channels and loudspeakers. It has further not heretofore been known to provide an electro-acoustical system for providing enhanced reverberation which has not been overly sensitive to room modes.
It is therefore an object of the invention to provide a method and apparatus for enhancing the diffusion of electro-acoustically developed reverberance.
It is a further object of the invention to provide a method and apparatus for enhancing reverberance diffusion which is not dependent upon a large number of loudspeakers.
It is a further object of the invention to provide an electro-acoustical reverberance enhancement system which provides an increased sense of naturalness, directionlessness and animation, which is not overly sensitive to room modes and which provides for facilitated microphone placement.